Selenium contained in wastewater drained from a desulfurization apparatus of a coal gasification combined power generation facility (integrated coal gasification combined cycle (IGCC)) or a desulfurization apparatus for treatment of flue gas from a boiler, is a hazardous substance under the Water Pollution Prevention Act of Japan, and it is strongly desired to remove it in a wastewater treatment process. It has been known that for a method of removing selenium like this, methods such as an iron coprecipitation method, an ion exchange resin adsorption method, a microbial treatment method, and the like may be used depending on the valence of the selenium (see Patent Literatures 1 to 3, for example). 0-valent selenium (e.g., selenium metal) and 4-valent selenium (e.g. selenite ions) are adsorbed and separated by an iron coprecipitation method or an ion exchange resin adsorption method. It is difficult to remove 6-valent selenium (e.g., selenate ions) by an ion exchange resin adsorption method or an iron coprecipitation method. Accordingly, 6-valent selenium is subjected to iron coprecipitation after reducing to 4-valent selenium.
However, wastewater treated in a coal gasification combined power generation process or a treatment f flue gas from a boiler in a reducing atmosphere includes selenium organic compounds and selenium in a plurality of forms. The effects of conventional removal methods cannot be obtained in treating such wastewater. For example, wastewater drained from coal gasification compound power generation facilities contain -2-valent selenium e.g., selenocyanate ions). It is difficult to remove such -2-valent selenium by conventional removal methods.